# Arc modules

> Write Arc code one time in a module, and import it into your automations.

A module is Arc code that automations and other modules import. A module has no Driver and does not run by itself.

## Create a module

1. In the Arcs toolbar, click **Create Arc automation** ().
2. Type a name, and select the **Module** type. The name can contain only letters, digits, and underscores.
3. Click **Create**.

## Export code

Put `export` before a top-level function, sequence, stage, or `:=` variable. Importers see only exported code.

```arc
export func psi_to_kpa() {v f64} f64 {
    return v * 6.895
}

export sequence press {
    stage fill {
        1 -> press_vlv_cmd
        tank_pt > 300 => next
    }
    stage hold {
        0 -> press_vlv_cmd
    }
}

export stage vent {
    0 -> press_vlv_cmd
    1 -> vent_vlv_cmd
}
```

## Import a module

The import path is the directory names and the module name, joined by `.`. This automation imports the module `tank` from the directory `lib`:

```arc
import lib.tank

tank_pt -> tank.psi_to_kpa() -> tank_pt_kpa
start_cmd == 1 => tank.press
abort_cmd == 1 => tank.vent
```

An import can also use [an alias or a group](https://docs.synnaxlabs.com/reference/control/arc/reference/syntax#imports).

## Edit a module

The controls bar shows “Compiled” or “Failed to compile” with the error.

A running automation keeps the module code that it started with until you redeploy it.

If you rename, move, or delete a module, each import of the old path fails with `unknown module "lib.tank"`.

## Rules

| Rule                                                              | Error                                                                |
| ----------------------------------------------------------------- | -------------------------------------------------------------------- |
| A module cannot have an `authority` declaration.                  | `a module cannot have an authority block`                            |
| A module cannot have a top-level flow.                            | `a module cannot have a top-level flow`                              |
| Only a module can use `export`.                                   | `export is only valid in a module`                                   |
| An importer can use only exported code.                           | `'scale' is not exported by module "lib.tank"`                       |
| You cannot import an automation.                                  | `"fill" is an automation and cannot be imported`                     |
| A module cannot import itself, directly or through other modules. | `import cycle: a -> b -> c -> a`                                     |
| A module outside a directory cannot have a standard library name. | `"time" is a standard module name and cannot go directly under Arcs` |